1. Field of the Invention
The present invention relates to a digital camera, more specifically relates to an AF (autofocus) digital camera capable of taking color images.
2. Description of the Prior Art
In AF digital cameras, a part of the light bundle of an object which is formed through a photographing lens is lead to an AF sensor unit to measure an object distance (defocus amount) at the AF sensor unit. FIGS. 7A, 7B and 7C are diagrams illustrating the principle of operation of such an AF sensor unit which is incorporated in an SLR type of digital camera. Each of these diagrams shows a state of the aforementioned part of the light bundle of an object, which is formed through a photographing lens, in the AF sensor unit, and further shows the distance (image-to-image distance) between two separate images received by an AF sensor 226 thereon (between two peaks of a signal output from the AF sensor). The above-mentioned part of the light bundle of an object, which is formed through a photographing lens, is separated into two light bundles by an aperture mask 224 having two apertures, which are in turn focused on the AF sensor 226 via two separator lenses 225, respectively. The AF sensor 226 is a line sensor, or the like, and measures the distance between two images of the two light bundles focused on the AF sensor 226. Thereafter, an in-focus state is achieved by driving a focusing lens system (focusing lens group) of the photographing lens by feedback control performed by an AF controller (focusing device (not shown)) so that the aforementioned image-to-image distance becomes a predetermined distance.
In this type of AF sensor unit, the effective focal length of the optical system of the AF sensor unit varies depending on differences in color of light of an object, i.e., differences in wavelength of light incident on the AF sensor unit, and therefore, the imaging position in the AF sensor 226 varies depending on wavelengths of the received light, which makes it impossible for the aforementioned focusing lens system of the photographing lens to obtain a correct in-focus state, i.e., which becomes a cause of an AF error. For instance, if the image-to-image distance Lg at the AF sensor 226 upon receiving green light is regarded as a reference distance (see FIG. 7A), the effective focal length of the optical system of the AF sensor unit increases as shown in FIG. 7B when the AF sensor unit 226 receives red light (long-wavelength light), and accordingly, the image-to-image distance Lr at the AF sensor 226 upon receiving red light becomes shorter than the reference distance, and the effective focal length of the optical system of the AF sensor unit decreases as shown in FIG. 7C upon the AF sensor unit 226 receiving blue light (short-wavelength light), and accordingly, the image-to-image distance Lb at the AF sensor 226 upon receiving blue light becomes longer than the reference distance. Note that the optical path difference of a light bundle of an object due to the difference of light wavelength is exaggerated in FIGS. 7A, 7B and 7C for the purpose of illustration. Such variations of the image-to-image distance that depend on wavelengths of the received light cause variations in measured distance value, thus becoming a cause of an AF error.
To overcome this problem of AF error, a technique of eliminating AF error such as noted above, which is caused by color difference in light of an object the object distance of which is to be measured, by measuring color of the object light and correcting the output of the AF sensor unit (i.e., a measured object distance value or an in-focus position) in accordance with the measured color has been proposed in Japanese unexamined patent publication 2003-241064. In this publication, the photometric sensor is provided on a part of the light receptive surface with an optical filter so that the photometric sensor measures the object color by receiving light which is passed through the optical filter. In order to correct the AF error in an effective manner, colorimetric (color measuring) elements are disposed to carry out color measurements at positions corresponding to distance measuring points on an object which is to be photographed, i.e., at positions corresponding to points on object images which are formed on an AF sensor via separator lenses in the AF sensor unit.
In the technique shown in the aforementioned publication, since the photometric sensor is constructed so that the color measurements are performed with a part of the light receptive surface of the photometric sensor, it can be said that the photometric sensor and a calorimetric sensor (which includes the aforementioned colorimetric elements) are formed integral with each other, and accordingly, it is considered that the color measurements may exert an influence upon a photometering operation.
More specifically, in the technique shown in the aforementioned publication, the photometric sensor measures the intensity of visible light over all wavelengths thereof, whereas the colorimetric sensor measures the intensities of red, green and blue lights by a plurality of photoreceivers of the calorimetric sensor which include spectroscopic color filters for red, green and blue lights. Since the colorimetric sensor performs color measurements for distance measuring points on an object to exercise AF control in the AF sensor unit such as the aforementioned feedback control, the calorimetric sensor influences the photometric values measured at the distance measuring points. Namely, when taking a picture with a camera, it is generally the case that the object distance is measured to bring a portion of the object which is to be photographed into focus; however, if the photometric value measured at this portion of the object is influenced by the operation of the colorimetric sensor, this results in the colorimetric sensor influencing the correct exposure at object. A solution to this problem is to separate the photometric sensor and the calorimetric sensor from each other; however, this inevitably requires two sensors. Specifically, in the case where the aforementioned type of photometric and calorimetric techniques is applied to a digital camera, it is necessary for the digital camera to be provided with a color temperature sensor (white balance sensor) for determining a standard of reference used for photographing white light as a true white, which requires not only a photometric sensor and a calorimetric sensor but also a color temperature sensor, i.e., three sensors in total. This increase in number of sensors complicates the overall structure of the digital camera.